Mayor Bloomberg is pushing hospitals to hide their baby formula behind locked doors so more new mothers will breast-feed.

Starting Sept. 3, the city will keep tabs on the number of bottles that participating hospitals stock and use — the most restrictive pro-breast-milk program in the nation.

Under the city Health Department’s voluntary Latch On NYC initiative, 27 of the city’s 40 hospitals have also agreed to give up swag bags sporting formula-company logos, toss out formula-branded tchotchkes like lanyards and mugs, and document a medical reason for every bottle that a newborn receives.

While breast-feeding activists applaud the move, bottle-feeding moms are bristling at the latest lactation lecture.

“If they put pressure on me, I would get annoyed,” said Lynn Sidnam, a Staten Island mother of two formula-fed girls, ages 4 months and 9 years. “It’s for me to choose.”

Under Latch On NYC, new mothers who want formula won’t be denied it, but hospitals will keep infant formula in out-of-the-way secure storerooms or in locked boxes like those used to dispense and track medications.

With each bottle a mother requests and receives, she’ll also get a talking-to. Staffers will explain why she should offer the breast instead.

“It’s the patient’s choice,” said Allison Walsh, of Beth Israel Medical Center. “But it’s our job to educate them on the best option.”

Lisa Paladino, of Staten Island University Hospital, said: “The key to getting more moms to breast-feed is making the formula less accessible. This way, the RN has to sign out the formula like any other medication. The nurse’s aide can’t just go grab another bottle.”

Some of the hospitals already operate under the formula lockdown.

“New York City is definitely ahead of the curve,” said Eileen DiFrisco, of NYU Langone Medical Center, where the breast-feeding rate has surged from 39 to 68 percent under the program.

Breast-feeding in the first weeks gives a baby a critical healthy start, many medical experts say. It helps the digestive system develop and protects the baby with the mother’s immunities. Nursing also helps the mother recover from childbirth.

But not everyone is convinced.

“They make formula for a reason, and the FDA makes sure it’s safe,” said Roxanne Schmidt, whose 14-month-old twins were fed with formula from birth. “Locking it up is just wrong.”